1. Field of the Invention
The invention is related to field of laser treatment of biological tissues and in particular to the control of the immediate theater of treatment above and adjacent to the target area of the tissue surface or skin.
2. Description of the Prior Art
Successful laser treatment of hypervascular cutaneous lesions such as port wine stains, hemangiomas and telangiectasias is based on selective photo coagulation of blood vessels by laser heating without inducing thermal injury to the epidermis. Protecting the epidermis from laser induced thermal injury may also be beneficial in treatment of facial rhytides.
One method to overcome non-specific thermal injury is to spray a short cryogen spurt of the order of several milliseconds directly onto the skin's surface immediately prior to laser irradiation as described in the copending applications Ser. No. 08/222,976, filed Apr. 5, 1994, now abandoned, and entitled "Apparatus And Method For Dynamic Cooling Of Biological Tissues For Thermal Mediated Surgery", and Ser. No. 08/870,467, filed on Jun. 6, 1997 and entitled "Method and Apparatus for Causing Spatially Selective Coagulation During Thermally Mediated Procedures," assigned to the same assignee as the present invention and which are incorporated herein by reference. The methodology described therein is hereinafter referenced as cryogen spray cooling.
In contrast to other procedures such as placing ice or a cold substrate such as a cooled sapphire window in contact with the skin, cryogen evaporation on the skin's surface allows a localized cooling of the epidermis without lowering the temperature of the targeted blood vessels. Although preliminary studies indicate that the thermal injury to the epidermis can be reduced or eliminated by cryogen spray cooling while still achieving the therapeutic effect, understanding the cryogen evaporation process both while in transit and at the skin's surface is essential for optimization of the treatment parameters, namely the cryogen physical properties, spurt duration as well as the design of the delivery system.
In particular, what needs to be considered is that the dependence of the radiometric surface temperature of human skin based on: (1) ambient humidity level; (2) spraying distance between the delivery nozzle and the surface; (3) cryogen boiling point; and (4) the geometry of the delivery nozzle or the spray formed by it. Each of these parameters must then be taken into account for devising control of the treatment theater immediately above the skin or in the vicinity of the laser irradiation site to maximize the control of laser treatment combined with cryogen spray cooling.
Water vapor present in the ambient atmosphere during cryogen spray cooling interacts with the cryogen to form ice crystals. Condensation may begin immediately after the cryogen is released from the delivery nozzle into the ambient atmosphere and continues until the cooling capacity of the cryogen is exhausted. Increased water vapor concentration in the ambient atmosphere results in higher rates of condensation. The thermodynamics of the ice crystal formation process is a diversion of the latent heat of evaporation from the relatively short lived liquid cryogen droplets into a cryogen-ice mixture in the solid phase that may exist in the skin surface for hundreds of milliseconds. Additionally, because temperature of liquid cryogen droplets injected from the nozzle into the ambient atmosphere is lower than that of ice crystals, and instantaneous heat flux at the skin surface may be reduced when water vapor is present. Inasmuch as ice crystal formation on tissue surface affects the heat flux, control of water vapor concentration in the ambient environment in the theater of exposure is essential for efficacious or controlled clinical application of cryogen spray cooling.
Therefore, the present invention is a device to control atmospheric water vapor concentration to limit the effects of ice crystal formation on the skin surface during cryogen spray cooling.